This invention relates to a method of reducing the effects of motion of objects in an image, in particular to a method of reducing the effects of motion in magnetic resonance imaging. The invention particularly relates to a method of compensating for patient motion to produce a focussed image.
Magnetic resonance imaging or MRI is a well known medical imaging technique. In essence the technique relies on the reaction of the magnetic moments of certain nuclei to applied magnetic fields. Protons and neutrons, the basic constituents of nuclei, posses magnetic dipole moments. In nuclei with an even number of protons and an even number of neutrons the net effect is no residual magnetic moment. However nuclei with uneven atomic number (or uneven atomic mass) have a net magnetic dipole and hence a magnetic moment. At room temperature in the absence of an external magnetic field one would expect to find a random orientation of magnetic moments in a medium.
In an MRI imaging system an intense magnetic field is applied to the area to be imaged. This field is applied in one direction, conventionally referred to as the z-direction. The effect of the applied field is to align the magnetic dipoles in the item being imaged. The dipoles do not all line up in exactly the same way however. The dipoles tend to adopt either an orientation lined up in the same direction as the field, referred to as parallel, or an orientation where the dipoles align opposite the field direction, the antiparallel orientation. At room temperature, due to the parallel state being slightly more energetically favourable, slightly more nuclei tend to adopt the parallel configuration than the antiparallel configuration. This results in a net overall magnetic moment for the medium, parallel to the applied field.
The coupling effects of the magnetic moment of the nuclei with the applied field does not cause an exact alignment of the nuclear moment with the applied field. Instead the magnetic moment processes around the applied field. The frequency of precession, called the Larmor frequency, is proportional to the strength of the applied field. The stronger the applied field the faster the rate of precession.
In effect one can consider that the dipole moments of the nuclei have aligned so there is a component of the moment in the z-direction and a component rotating in the x-y plane at the Larmor frequency. As mentioned, throughout the whole object being imaged there is a greater component parallel to the z-direction than antiparallel so there is a net moment for the object. However the components in the x-y plane are still randomly arranged in the presence of a single field so there is no net moment in the x-y plane.
Applying an RF magnetic field at the Larmor frequency perpendicular to the applied field causes the dipoles to tip into the transverse, or x-y, plane. It also causes alignment of the dipoles. The net result is then a net magnetic moment in the x-y plane rotating at the Larmor frequency.
When the RF field is removed this net magnetic moment can be measured due to the inductance caused in receiver coils. Of course once the RF field is removed the net magnetisation of the item being imaged will start to revert to what it had been as the magnetic moments of the nuclei begin to align with the z-direction again.
There are two separate decay processes that occur. The first is the increase in the z-direction component of overall magnetic moment. Thus is sometimes referred to as longitudinal or spin axis relaxation and is due to the transfer of energy between excited nuclei and the lattice, or nearby macro-molecules. The second process, which is independent of the first, is that the precession of the moments of the nuclei, which had been brought into in phase by the transverse rf field, start to de-phase reducing the x-y component. The de-phasing process. known as transverse relaxation or spin-spin interaction is due to transfer of energy between nuclei in different states and also from magnetic field inhomogenities. In both decay processes the different types of material present in an object, say the differing types of tissue in a patient, will affect the relaxation processes. Hence measuring the differing strengths of signal received by the different types of tissue will reveal contrast in an image.
In order to form an image it is necessary to encode the dipoles of the signals emitted by the nuclei after magnetization have information regarding to the spatial positioning of those nuclei. The imaging processes can usually be described in the following terms. First of all is the step of selecting an image slice, i.e. a small volume to be imaged, and then spatially encoding the magnetic resonance signal emanating from that slice. The basis for this is that the frequency at which a nucleus resonates, its Larmor frequency, is a function of the strength of the static magnetic field in which it is located. Therefore by altering the strength of the magnetic field as a function of position, i.e introducing a magnetic field gradient, the Larmor frequency will also vary as a function of position.
Typically therefore a weak magnetic field that changes linearly with position is superimposed on the main static field to create a magnetic field gradient along the z-direction. An RF pulse with a narrow range of frequencies is then applied transversely. Only those nuclei whose Larmor frequency matches the frequency of the applied RF pulse will actually absorb the RF energy and undergo the tipping and alignment described above. Therefore by a careful choice of RF frequency only a narrow band or slice of the object being imaged will be excited.
Having selectively excited a slice of the object to imaged it is necessary to achieve spatial resolution within in a slice. Spatial resolution in one dimension, say the x-direction, can be achieved through use of a frequency encoding gradient. Immediately following the RF excitation pulse all spins of the nuclei of interest within the selected slice will be processing at the same frequency. Application of an additional gradient, orthogonal to the z-direction gives spatial resolution in one dimension. This additional gradient, known as a frequency encode gradient, will alter the Larmor frequency of the spin precession across the slice and allow spatial resolution.
Note that for medical MRI the nuclei of interest is almost exclusively the nucleus of hydrogen. However other nuclei species could be of interest in certain applications.
To get two dimension resolution across the slice it is necessary to use a phase encode step as well. Here following the RF excitation pulse a phase encoding gradient is applied in the y-direction for a short time. Remember that immediately following the RF excitation pulse all the spins in the selected slice will be in phase and processing at the same frequency. If a phase encode gradient is applied in the y-direction the spins will have their resonant frequencies, and hence the rate of procession, altered according to their position along the y-direction. When the phase enode gradient is removed all nuclei in the slice will again be subject to the same static field strength and hence the spins will again start to precess at the same frequency. The effect of the phase encode gradient will have been to alter the phase of the spins according to their position along the y-axis in a known manner. The frequency encode gradient may then be re-applied.
The measured signal at a particular frequency (and therefore position along the x-axis) is the sum of all the vector contributions from a row of spins in the y-direction. The actual signal measured of course is a composite of all the frequency components along the x-axis.
To generate an image during the time that the frequency encode gradient is applied the signal is sampled Nx times yielding a pe-line, which is a vector or line of data having Nx points. Repeating the measurements Ny times for differing values of the y-gradient yields a matrix of Nxxc3x97Ny amplitude points. In general to generate a final image of Nxc3x97N pixels the phase encoding step must be repeated N times with different values of the phase encoding gradient.
Other acquisition schemes for slice or volume imaging exist which gather the data in different ways, however the basic principles remain the same. Also there are a number of different imaging sequences that can be applied. However in all cases the raw data collected exists in what is called k-space.
To obtain the image a Fourier transform is performed along both the x and y axes to produce a 2D spectrum of data points whose intensity is representative of signal distribution in the imaging slice.
Patient movement during the acquisition of MRI images results in degradation of the images that can obscure the clinically relevant information. Each readout period takes a few milliseconds, i.e readout of the series of Nx data points (known as a phase encode line), whereas the time interval between readouts, i.e. the next value of phase encode gradient, might be between 100 and 400 ms. The majority of blurring and ghosting artefacts caused by patient motion are due to motion between lines in k-space, rather than motion during a single readout.
Movement leads to errors between lines of k-space which, in the resulting image, appear as blurring and ghosting, which may be in the phase encode (pe) and frequency encode (fe) directions. These errors can result from translational movement in the pe and fe directions and also rotational movement. Translations of the patient in the readout direction result in a frequency dependent phase shift in each line of k-space. Rotations in the spatial domain are also rotations in k-space and result in k-space changes that are more complicated function of position in k-space.
Various techniques have been employed to try to correct for image artefacts introduced into an image through motion. However most of the techniques known for correcting for patient motion involve a modified signal acquisition technique which may involve additional scans or even additional equipment.
International Patent Application WO98/01828 discloses a technique for reducing the effect of motion induced artefacts in an image using purely post data gathering signal processing effects. In the technique described therein the data is manipulated to counteract possible movement induced artefacts and the manipulated data compared using a focus condition to see if the image quality is improved. This technique can involve a large amount of processing due to the need to perform a high dimensional search in motion parameter space. Furthermore the method may involve the grouping of k-space lines to more accurately determine motion parameters, although this grouping can decrease the temporal resolution of the motion found.
Another method of correcting for motion induced image artefacts in the method of Projection onto Complex Sets (POCS) Hedley M, Hong Y and Rosenfeld D. xe2x80x9cMotion Artifact Correction in MRI using generalized projectionsxe2x80x9d IEEE Trans. Med. Imag., 10:40-46, 1991. This is a method whereby a good quality image is used to form a binary mask. The mask defines the tissue-air boundary, i.e. outside the mask there should be no signal. Motion induced artefacts in the acquired image cause apparent signal in the air. The POCS method sets all outside the mask in the acquired image to black. The image data is then Fourier transformed to k-space. A new complex k-space is formed from the modulus of the measured data and the phase of the estimation from the previous step. This new k-space is Former transformed to the image domain and the process iterates. This method however involves a large amount of Fourier transformation as the process iterates and hence involves a large amount of computational effort and hence time. Further the method requires the spatial alignment of the binary mask with the acquired image before processing which is not always possible to achieve.
The present invention seeks to provide an alternative method for correcting for motion induced artefacts in an image.
Thus according to the present invention there is provided a method of producing an image of a scanned object corrected for artefacts introduced by unwanted motion of said object during the scan comprising the steps of taking a k-space image data set comprising a number of data points derived from the object scan, forming an first data set from some of the k-space image data points, adding at least one additional data point to the first data set to form a second data set, predicting, from the first data set, the at least one added data point, comparing the at least one added data point with the predicted at least one data point to determine the extent of any motion and correcting the at least one added data point to compensate for any detected motion.
An MRI image, as discussed above, is generally collected as an array of data points in k-space. Each k-space data point contributes to the whole image. Thus any set of k-space data points, which is not undersampled and includes zero spatial frequency, can be used to form an image. The resolution of the image is increased as the number of k-space space data points used to form that image is increased.
The present invention resides in the fact that a lower resolution image, formed by a sub-set of k-space data points, can be used to predict what a next data point or set of points would be. A data point or set of data points may be conveniently termed a data vector. The method according to the present invention is therefore what may be termed a direct focussing method wherein the extent of any motion is determined directly through calculation. This greatly reduces the amount of processing required as compared with focussing methods using focus criteria which require large numbers of high dimensional searches to be performed in motion parameter space. This offers advantages not only in speed but also offers a greater degree of robustness in the focussing method in that focussing methods utilising focus criteria can converge to local rather than global minima, thus giving bad estimates of motion.
The prediction could be carried out using the raw k-space first data set. Alternatively the prediction could be carried out in image space using an image formed from the first data set. It is noted here that the term image is used to refer to the complex data set obtained by performing a two dimensional Fourier transform (Fr) on the k-space data set. The actual visual image used is obtained by the amplitude of the real part of this image.
As a further alternative the prediction could be performed in hybrid space. Hybrid space is obtained by Fourier transforming the k-space date set in one direction only. Different hybrid spaces are obtained by performing the Fr in either the frequency encode (FE) direction or the phase encode (PE) direction. Conveniently FE transformed hybrid space could be used.
Performing the prediction on the raw k-space data removes the need for any Fourier transforms to be performed and offers the quickest processing. However as the k-space data varies to a greater extent than image space or hybrid space accurate prediction is harder. Complex image space can be used but use of hybrid space can offer good predictability without undue processing requirements.
The invention is based on the fact that, as the resolution of an image is increased by adding data points to the k-space data, these new data points added are largely predictable from the data set existing before the new data was added, assuming no patient motion occurred while the new data was added.
Motions of the patient occurring during the acquisition of new line of phase encode data translate to phase ramps on that pe line which can be noticeable in the image or the data in k-space or hybrid space. The predicted complex data added is preferably compared with the actual phase of the added data and the difference between them used to determine the extent of any motion. Any displacements present in the frequency encode direction may be determined before determining any displacement in the phase encode direction. Conveniently then the displacement in one direction may be compensated for before determined the extent of any displacement in the other direction.
The present invention is preferably used inductively. A focussed low resolution image is used as the first data set and data points added thereto to form the second data set. Once the added data points have been corrected to compensate for any detected displacements the focussed second data set can be used as the staffing point, i.e. the first data set, in the next iteration. One advantage of the present invention is that only displacement associated with those points added need be considered as the first data set is focussed.
Preferably the first data set comprises a number of phase encode lines. A phase encode line represents a series of data points taken after a particular phase encode gradient was applied. As the data points making up a phase encode line are typically taken in a fraction of a second there will not normally be significant distortion intra line due to object motion. Therefore one can concentrate purely on the motion increment between successive phase encode lines, which can be assumed near constant for the whole line.
The data points added to the first data set to form the second data set are also conveniently made up of a number of phase encode lines. As mentioned above the phase encode line can be considered to have minimal motion effects intra-line. Therefore adding a whole pe line or more than one line at a time is a convenient way of building up to the full resolution image. However a complete set of phase encode lines covering all spatial frequencies in an image can take tens of seconds to gather which can result in the need to compensate for object motions between lines. It is possible however to add smaller groups of data points or even a data point at a time if required however.
In order to make prediction easier and maximise accuracy the phase encode lines arm preferably added one at a time.
When the first data set comprises only a very few phase encode lines it is more difficult to predict what the added data points may be as there is less information for the prediction. Therefore a first data set comprising a certain number of phase encode lines, say 8 or 10, may be taken as the starting first data set and phase encode lines added one at a time, or in pairs, symmetrically about DC, to build up to a full resolution image. This starting first data sat image may be initially focussed using some other means, such a focus criterion. The starting first data set is preferably chosen to be symmetrical about DC. The additional phase encode lines can then be added sequentially alternating on either side of DC. DC is point which corresponds to no phase encoding gradient applied and no frequency encode gradient, i.e. the zero or centre point in k-space. It corresponds to zero spatial frequency in the PE and FE directions, in the raw data space. It should of course be noted that the phase encode lines need not have been acquired sequentially in time. It is best however if the image is built up sequentially in terms of k-space lines as xe2x80x98holesxe2x80x99 in the block of k-space lines would be similar to an undersampled data set and could result in artefact in the image.
The term phase encode line here should not been seen as limiting. It will be apparent to the person skilled in the art that the methodology is also applicable to non Cartesian data sets, such as generated in polar-type acquisitions. By the term phase encode line is simply meant a set of data points taken at one particular phase encode gradient value.
One way of determining displacements in the frequency encode direction is to perform a ID Fourier transform along the Frequency encode direction on the firs k-space data set to form an hybrid space data set. The hybrid space data set is then analysed to predict what the next line of hybrid space data would be. The hybrid data set together with the predicted data are then transformed back into k-space to provide a predicted line in k-space. A pointwise ratio of the predicted k-space line to the actual k-space line added is then formed and the amount of linear phase variation used as an estimation of displacement in the FE direction. The added data is then corrected to compensate for the determined FE displacement. Alternatively the vector ratio of the predicted k-space line and actual added line is Fourier transformed and theposition of the frequency encode spectral maximum relative to DC used to give an estimation of the displacement in the FE direction.
In order to then compensate for any displacements in the PE direction the phase of the frequency encode spectral maximum relative to DC is used as an estimation of displacement in the PE direction.
Preferably the object scan is performed with a magnetic resonance imager.
Conveniently the method of the present invention is implemented as a computer programme for automatic focussing of an MRI image.